Coating materials, e.g., polymers, are often used as protective barriers on circuit boards, electrical components, medical devices and the like. Parylene is a generic term often used to describe a class of poly-p-xylylenes which are derived from a dimer of the structure: ##STR1## where X is typically hydrogen or a halogen, e.g., fluorine, and A and B, when present, are halogens, e.g., chlorine. Due to its ability to provide thin films and conform to substrates of varied geometric shapes, parylene is ideally suited for use as a conformal coating.
Typically, parylene is applied by vapor deposition under vacuum conditions wherein the parylene monomer is condensed and polymerized directly on the surface of the object to be coated. Since the parylene monomer is not stable, the parylene dimer, as illustrated above, is used as the starting material.
Typical apparatus for carrying out parylene vapor deposition coating processes comprise a vaporization zone, wherein the parylene dimer is vaporized, a pyrolysis zone, wherein the parylene dimer is pyrolyzed to its monomeric form, a deposition chamber, wherein the objects to be coated are exposed to the parylene monomer, and a vacuum means for maintaining vacuum conditions within the deposition chamber.
Apparatus such as described above have been particularly useful for coating the external surfaces of objects. However, apparatus are also desired for coating the inside surfaces of hollow objects with a coating material, e.g., parylene, by vapor deposition.